Queen. because she got the best movement to do everything.

Ofcourse queen is really flexible. But what do you mean exactly with reliable? I would say king. You can decieve and outplay easily with good king placements. Not gonna write anything long here but basicly sure you do the most movements with everything except the king but trust me, fooling your opponents by moving your king oddly much is a quite good tactic. If you're doing it right that is.

I would go with pawn because it plays a really important role in both attack and defense. It is kind of weak and defenceless when its alone, thats why you have eight of them. Also every single pawn can be turned to a queen when it reaches the end of the board.

It's not a matter of what chess pieces you have, it's how you use them.

But if the question was which is your favourite, then I'd say the horses. I like forcing checkmate with just 4 horses.